Box office upsets like this are rare, since studios are generally aggressive in their Sunday estimates.

“Hangover,” however, grossed $1.7 million more than projected, even with the NBA playoffs and MLB baseball games.

Todd Phillips directed the buddy comedy, which stars Bradley Cooper, Ed Helms, Zach Galifianakis and Justin Bartha as four friends who have an unforgettable trip to Las Vegas.

Early Sunday, Warners estimated “Hangover” would gross $11.9 million for the day; instead, the film grossed $13.6 million. The comedy grossed $16.5 million on Friday and $14.9 million on Saturday.

The size of “Hangover’s” opening is one of the biggest surprises of the year.

Generally speaking, R-rated comedies don’t open as big, but rather, have long runs. “The Hangover” enjoyed the third best debut ever for an R-rated laffer after “Sex and the City” ($57 million) and “American Pie 2.”

The film earned a pure “A” CinemaScore , meaning it earned an A in every category during exit polling.

“Sunday’s performance on Sunday is a real indication that word of mouth has already started to propel to the movie to a point where we are going to have a big sleeper and long-running hit,” Warners prexy of domestic distribution Dan Fellman said.

Fellman said the film dropped only 12% from Saturday to Sunday, an unusually low decline.

Unlike “Hangover” and “Up,” Universal’s Will Ferrell adventure-comedy “Land of the Lost” didn’t participate in the weekend B.O. bounty.

The film is the summer’s first major disappointment, opening to $18.8 million, well below the $30 million U had hoped for. On Sunday, U estimates had “Land of the Lost” opening to $19.5 million.

“Land of the Lost” cost at least $100 million to produce and was co-financed by Relativity Media.

Also on Monday, shares of Imax fell 4.32% to $7.31 over concerns that Imax isn’t debuting Warner Bros.’ “Harry Potter and the Half-Blood Prince” until two weeks after its bow in regular theaters on July 15. The reason for the delay is that Imax was already committed to a five-week run of Paramount’s “Transformers: Revenge of the Fallen” when Warners decided to push back the release of “Half-Blood Prince” to summer 2009.

A portion of “Half-Blood” was shot in 3-D with Imax large format cameras. Imax and Warners say fans of the Harry Potter franchise will still be eager to see the film in Imax, even if that means two weeks later.